An observer by the name of Barkhausen identified Barkhausen magnetic domain shift phenomena in about 1917. This phenomena is a transition which occurs within a ferromagnetic specimen during magnetization. The phenomena description postulates that the specimen is formed of innumerable small magnetic domains. A domain is a three-dimensional portion of the specimen, which particular portion is deemed to act as a small magnet initially postured in a randomly oriented position within the specimen. The domains are randomly distributed and positioned when the specimen is in a nonmagnetic state. The three-dimensional integration of the magnetic force formed in bulk by the specimen is nil as a direct result of the random orientation of the various domains within the specimen.
When a magnetic flux is applied to ferromagnetic material, the flux forces reorientation of the domains. Thus, the domains are observed to shift suddenly. The shifting is cumulative, as first one domain and then another shifts as they are aligned with the magnetic flux direction. Shifting and change in domain size occur both separately and together. As an example, one domain may only rotate. Another will expand as its borders (walls) move which expansion will inevitably reduce the volume of adjacent domains. Shifting of domain walls occurs suddenly, accompanied with an acoustic wave and a change in magnetic intensity in the specimen. As the flux increases, the number of domains aligned with the lines of flux increases. Each shift of a domain is accompanied by magnetic perturbation indicative of the shift. So to speak, the shift by the domain is a reorientation of the domain and may be likened to rotation of a small magnet within a ferromagnetic material. When the small magnet is rotated, it induces a changing field in the adjacent ferromagnetic material, and it is possible to measure this phenomena externally. Indeed, the shift phenomena is observed by review of the magnitude of the Barkhausen signal, the signal being the amplitude of the envelope of the distribution of voltage pulses induced during a change in magnetization. This method of detection is called the Barkhausen magnetic effect.
The Barkhausen magnetic effect is stress dependent. Speaking generally, for materials with a positive magnetostrictive coefficient, the activity increases when the specimen is under tension and decreases when the specimen is under compression. Thus, Barkhausen magnetic signal strength is a measure of the nature and magnitude of the stress occurring in the test specimen at the moment of magnetization. The present invention overcomes some limitations of induction-type Barkhausen magnetic test apparatus described in U.S. Pat. No. 3,427,872. Although domain walls located throughout the entire magnetized volume of a specimen undergo movement when the applied magnetic field strength is varied, only those domain wall movements near the surface can be detected inductively since those deeper within the specimen are electromagnetically shielded. So to speak, the signal strength is shielded by the specimen, itself, so that Barkhausen measurements obtained inductively show only surface occurrences, which data is therefore incomplete. In many instances, stress distribution is not necessarily even or uniform. Given a particular specimen, the stress may concentrate in the central portions of material and not manifest itself near the surface. Because inductive detection of Barkhausen phenomena is possible only at the surface, the surface manifestation must inevitably be the only measure available.
The present disclosure is concerned in the main with utilization of the Barkhausen acoustic phenomena for repeatable nondestructive testing of a specimen. It has been discovered that the sudden movement of magnetic domain walls creates a unique vibratory signal which travels through the specimen. The direction of propagation appears to include all possibilities so that domain wall shifts, even those deep within a specimen, produce vibrations which travel to the surface, probably all surfaces. Barkhausen acoustic sounds thus occur with movement of the various domain walls in the specimen. The shifts so occurring are readily detectable at the surface by an acoustic transducer. It has been found that Barkhausen acoustic signal is dependent on residual and applied stress existing in the specimen as well as the characteristic of the microstructure of the specimen. Thus, utilizing appropriate scale factors, the Barkhausen acoustic phenomena will form an indication of the stress level of microstructure in encompassing regions of the specimen extending from the surface to the interior.